The present invention relates to the field of acousto-optical scanners, especially those capable of generating fast non-linear scans, and/or fast longitudinal focus scanning.
Acousto-optic scanners (AOS""s) have no moving parts and are thus capable of scanning laser beams much faster than mechanical scanners. The limitation on the scanning speed of acousto-optic scanners arises from the transition or access time Taccess of the acoustic wave across the width of the laser beam. In most present applications, the requirement is for a scan angle xcex1 which is linear with time xcex1(t)=at. This is achieved by linear chirping of the acoustic wave frequency f(t). For such linear scans the scan rate can approach 1/Taccess, since the effect of a linear chirp can be described as an effectively constant, time independent cylindrical lens, as shown by A. VanderLugt, in the book xe2x80x9cOptical Signal Processingxe2x80x9d published by John Wiley and Sons, 1992. Because of its time independence, such a lensing effect can be readily compensated for, by the addition of an external lens of identical and opposite power. There is consequently virtually no reduction in the number of resolvable points (NRP) obtainable from the scanner. This is shown in the article entitled xe2x80x9cDesign relationships for acousto-optical scanning systemsxe2x80x9d by A. VanderLugt, and A. M. Bardos, published in Applied Optics, Vol. 31, pp. 4058-4068 (1992).
In order to determine the limitations on the performance of such AOS""s, the deflection of a uniform laser beam with a diameter D and a wavelength xcex is considered. Though the ensuing analysis deals with the case of a uniform laser beam, it is readily adapted to other beam shapes, with small changes in the numerical constants. The beam is Bragg deflected by a perpendicular acoustic wave with frequency f(t) and velocity xcexd in an acousto-optic element. The angular scan-span in the first diffraction order is given by xcex94xcex1=xcex94fxcex/xcexd, where xcex94f is the acoustic frequency span. Dividing xcex94xcex1 by the diffraction limited angular spread xcex/D yields the so called static, or low scan-rate NRP as:
NRPstatic=xcex94fD/xcexd=xcex94fTaccess.xe2x80x83xe2x80x83(1)
Equation (1) indicates that to achieve a large value of NRPstatic, large values of xcex94f and Taccess are required. Scanners with xcex94f of more than 100 MHz are typically very expensive and suffer from reduced diffraction efficiencies and increased acoustic-wave absorption and heating. Hence, scanners with large NRPstatic mostly rely on the use of large values of Taccess, which is achieved by a combination of a slow acoustic velocity (using shear-mode acoustic waves) and a large laser beam diameter.
Next, the effects of the scan time, Tscan, on the resolution are included. For a linear frequency chirp, which, as mentioned above, acts as a constant focal length cylindrical lens, Tscan can approach Taccess, and hence the dynamic (or fast-scan) resolution limit for linear scans is simply given by:
NRPdynamic,linearxcx9cxe2x89xa6xcex94fTscan.xe2x80x83xe2x80x83(2)
However, for some applications, it would be useful to have non-linear scans, namely scans with non-constant rate or a variable span. Non-linear acousto-optic scanners have recently attracted much attention for a variety of such applications. They have been used for generating two-dimensional circular scans to form dark optical dipole traps for ultra cold atoms, as described by N. Friedman et al, in Physical Review A, Vol. 61, page 031403(R) (2000), for stirring Bose-Einstein condensates, as described by R. Onofrio et al, in Physical Review Letters, Vol. 84, p. 810 (2000), and for rotating Bose-Einstein condensates, as described by K. W. Madison et al, in Physical Review Letters, Vol. 84, p. 806 (2000).
In addition, they are almost essential for ultra-fast laser vector plotters where arbitrary (and hence non-linear) scans are required to efficiently plot sparse information over a large area. For example, in order to plot a ring whose line width is 1000 times thinner than its diameter, 1000xcfx80 resolvable points are required using a vector-plotter with a circular scan, as compared to 1,000,000 resolvable points using a conventional two-dimensional raster-mode scanner. Other applications include ultra-fast switching use in optical communication networks.
However, such non-linear scans must inherently use a non-constant frequency chirp, and since the focal length of the effective cylindrical lens is proportional to the chirp rate, the resulting effective lens is thus of constantly changing power, and so cannot be simply compensated for by the addition of an external lens. The result is a high level of aberrations and a drastically reduced NRP for fast scans.
An analysis of the combined limitations on speed and resolution for a non-linear AOS, similar to that performed above for the linear case, shows that such non-linear AOS""s are indeed significantly inferior to linear scanners in these respects, as is now shown hereinbelow.
For arbitrary or totally random scans, which include NRPdynamic resolution points in a random order, Tscan must be NRPdynamicxc3x97Taccess. For a given scan time, there is an optimal value for the access time, (Taccess)opt.=(Tscan/xcex94f)xc2xd, which results in a significantly worse limitation on the optimal resolution than that for a linear scan:
NRPdynamic,randomxcx9cxe2x89xa6(xcex94fTscan)xc2xd.xe2x80x83xe2x80x83(3)
Using typical values of parameters of xcex94f=100 MHz and Tscan=10 xcexcsec, the linear scan has an NRP value of the order of xcx9c1000 as compared to a value of the order of xcx9c30 for the random scan. Such a level of NRP is unacceptable for many high resolution applications.
There therefore exists a serious need for a acousto-optical scanner capable of performing high-speed, non-linear scanning, while maintaining levels of NRP which are close to those typically attainable with equivalent linear scanners of similar specifications.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.
The present invention seeks to provide a new acousto-optic scanner capable of high scanning speed, by the use of two acoustic waves with the same frequency modulation, propagating in opposite directions through one or more acousto-optical media disposed in the path of the beam to be scanned. This scheme preferably completely suppresses the linear frequency chirp, and thus enables the generation of fast non-linear scans and non-constant linear scans, with only a limited reduction of the NRP as compared to the linear scan case. In addition, by changing the phase between the modulating signals, such a scanning system also preferably provides fast longitudinal scans of the focal point of the beam along the optical axis. The use of two counter propagating acoustic waves with the same frequency modulation, is applicable for providing one-dimensional scans, but the method can be readily generalized to two-dimensional scans by cascading two of such one-dimensional scanners orthogonally. One and two dimensional non-linear scans can also preferably be obtained with two and four acoustic transducers, respectively, attached to a single crystal. Three dimensional scans can preferably be obtained by means of combinations also involving scans of the center frequencies of the frequency modulated acoustic waves in the previously described embodiments.
There is further provided, in accordance with a preferred embodiment of the present invention, an acousto-optic scanner consisting of at least one acousto-optic element disposed in the path of a beam to be scanned, the element supporting at least two frequency-modulated, counter-propagating acoustic waves, such that the frequency chirp across the beam is essentially suppressed.
In the acousto-optical scanner described above, the counter-propagating acoustic waves may have essentially the same frequency modulation.
Furthermore, the at least one acousto-optic element may consist of at least two acousto-optic elements, each of the elements supporting one of the counter-propagating acoustic waves.
There is also provided in accordance with yet another preferred embodiment of the present invention, an acousto-optical scanner as described above, and wherein the essential suppression of the frequency chirp enables the generation of a fast, non-linear scan, or of a linear scan with non-constant parameters.
In accordance with still other preferred embodiments of the present invention, the acousto-optical scanner as described above may produce a scanned beam essentially free of aberrations arising from the first order chirp, and may also essentially eliminate longitudinal scan.
There is further provided in accordance with still another preferred embodiment of the present invention, a two-dimensional acousto-optical scanner consisting of at least one acousto-optic element disposed in the path of a beam to be scanned, the element supporting at least two sets of acoustic waves mutually angularly disposed to each other such that the beam is scanned in two dimensions, each set consisting of frequency-modulated, counter-propagating acoustic waves, such that the frequency chirp across the beam is essentially suppressed. Two of the at least two sets of acoustic waves may be essentially orthogonal.
In the two-dimensional acousto-optical scanner described above, each of the sets of counter-propagating acoustic waves may preferably have essentially the same frequency modulation.
Furthermore, the at least one acousto-optic element may preferably consist of at least two acousto-optic elements, each of the elements supporting at least one set of the counter-propagating acoustic waves.
In accordance with further preferred embodiments of the present invention, in the two-dimensional acousto-optical scanner described above the essential suppression of the frequency chirp may preferably enable the generation of a fast, non-linear scan, or of a linear scan with non-constant parameters.
There is provided in accordance with yet a further preferred embodiment of the present invention, a two-dimensional acousto-optical scanner as described above, and wherein the scanned beam is essentially free of aberrations arising from the first order chirp. Additionally, the two-dimensional acousto-optical scanner may preferably essentially eliminate longitudinal scan.
There is even further provided in accordance with a preferred embodiment of the present invention, an acousto-optic scanner consisting of at least one acousto-optic element disposed in the path of a beam to be longitudinally scanned, the element supporting at least two frequency modulated, counter-propagating acoustic waves, two of the waves having a phase angle between them such that the scanner provides a longitudinal scan of the focal plane of the beam. The phase angle may preferably be essentially xcfx80 radians.
Furthermore, in accordance with yet another preferred embodiment of the present invention, in the acousto-optical scanner described above, the at least one acousto-optic element may be at least two acousto-optic elements, each of the elements supporting one of the counter-propagating acoustic waves.
In accordance with yet another preferred embodiment of the present invention, the acousto-optical scanner described above may essentially eliminate lateral scan, and it may also produce the optical effect of a variable focus cylindrical lens.
There is further provided in accordance with yet more preferred embodiments of the present invention, an acousto-optical scanner as above, and wherein the counter-propagating acoustic waves have essentially the same frequency modulation or wherein the at least one acousto-optic element consists of at least two acousto-optic elements, each of the elements supporting one of the counter-propagating acoustic waves.
In accordance with still another preferred embodiment of the present invention, there is provided an acousto-optic scanner consisting of at least one acousto-optic element disposed in the path of a beam to be longitudinally scanned, the element supporting at least two sets of frequency modulated, counter-propagating acoustic waves, mutually angularly disposed to each other, two of the waves having a phase angle between them such that the scanner provides a longitudinal scan of the focal point of the beam, without any lateral displacement. Preferably, two of the at least two sets of acoustic waves are essentially orthogonal. Additionally, the phase angle is preferably essentially xcfx80 radians.
There is further provided in accordance with still another preferred embodiment of the present invention, a three-dimensional acousto-optical scanner consisting of at least one acousto-optic element disposed in the path of a beam to be scanned, the at least one element supporting at least two sets of acoustic waves, mutually angularly disposed to each other such that the beam is scanned in two dimensions, each set of acoustic waves consisting of frequency-modulated counter-propagating acoustic waves, and wherein the center frequencies of each of the two frequency-modulated counter-propagating acoustic waves are scanned, and wherein the counter-propagating acoustic waves in the sets have a phase angle between them such that the scanner provides a three dimensional scan of the focal point of the beam. Preferably, two of the at least two sets of acoustic waves are essentially orthogonal. Additionally, the phase angle is preferably essentially xcfx80 radians.
In accordance with a further preferred embodiment of the present invention, there is also provided an acousto-optic scanning system consisting of at least one acousto-optic element disposed in the path of a beam to be scanned, a generator for supplying frequency modulated drive signals such as to propagate acoustic waves in the at least one acousto-optic element, and a focusing element for focusing the scanned beam, and wherein the acoustic waves comprise at least two frequency-modulated counter-propagating acoustic waves, such that the frequency chirp across the beam is essentially suppressed.
There is provided in accordance with yet further preferred embodiments of the present invention, acousto-optic scanning systems, whether providing one dimensional or two dimensional lateral scans, or longitudinal scans, or combinations of both in order to provide three-dimensional scans, and which include any of the acousto-optic scanners described hereinabove.
There is even further provided in accordance with a preferred embodiment of the present invention, a method of generating a fast, non-linear scan of an optical beam, consisting of the steps of disposing at least one acousto-optic element in the path of the beam, and generating within the element at least two frequency-modulated counter-propagating acoustic waves, such that the frequency chirp across the beam is essentially suppressed.
In accordance with yet another preferred embodiment of the present invention, there is provided a method of generating a fast, longitudinal scan of the focal plane of an optical beam, consisting of the steps of disposing at least one acousto-optic element in the path of the beam, and generating within the element at least two frequency-modulated counter-propagating acoustic waves, two of the waves having a phase angle between them such that the scanner provides a longitudinal scan of the focal plane of the beam.
There is further provided in accordance with still another preferred embodiment of the present invention, a method of generating a fast, longitudinal scan of the focal point of an optical beam, consisting of the steps of disposing at least one acousto-optic element in the path of the beam, and generating within the element at least two sets of frequency-modulated counter-propagating acoustic waves, mutually angularly disposed to each other, two of the waves having a phase angle between them such that the scanner provides a longitudinal scan of the focal point of the beam without any lateral displacement.
In accordance with another preferred embodiment, there is also provided a method of generating a three-dimensional scan of the focal point of an optical beam, consisting of the steps of disposing at least one acousto-optic element in the path of the beam, and generating within the element at least two sets of acoustic waves, mutually angularly disposed to each other, each set of acoustic waves consisting of frequency-modulated counter-propagating acoustic waves, scanning the center frequencies of each of the two frequency-modulated counter-propagating acoustic waves, and wherein the counter-propagating acoustic waves in the sets have a phase angle between them such that the scanner provides a three dimensional scan of the focal point of the beam.